During borehole drilling, produced drill cuttings need to be transported to the surface in order to prevent their accumulation at the bottom of the borehole. Borehole cleaning requires efficient and fast removal of the cuttings, as inadequate borehole cleaning could cause significant drilling problems such as formation fracturing, excessive torque and drag. These can result in an increase in drilling costs.
Borehole cleaning is primarily achieved by the circulation of a drilling fluid. The drilling fluid removes the drilled materials from the borehole by carrying them to the surface. It also lubricates and cools the drill bit and stabilises the borehole by maintaining hydrostatic pressure within the borehole in order to prevent formation fluid entering the borehole.
Due to its role, various properties of the drilling fluid such as flow rate and annular fluid velocity, mud density and viscosity, and pressure changes have a strong effect on the efficiency of the borehole cleaning process, which is further influenced by the inclination angle of the borehole and properties of the cuttings.
By monitoring properties of the drilling fluid and the amount of cuttings brought to the surface, information can be obtained on the efficiency of the drilling and the rate of borehole cleaning.
One way to monitor the efficiency of borehole cleaning is to weigh the well cuttings that have passed through a shale shaker and estimate a volume of cuttings based on the measured weight. The volume can be compared with the theoretical volume of the borehole.
GB-A-2363847 discloses measuring fluid density and determining hole cleaning problems using differential pressure measurements. In a first embodiment two spaced apart sensors on the riser allow for a determination of the density of the well fluid, which is indicative of borehole cleaning efficiency. In a second embodiment a further set of sensors are provided to measure the drilling fluid as it enters the well.